1. Field of the Invention
The present invention relates to a regulating system for controlling the concentration of nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO) in conjunction with the cleansing of emission gases, and also to a device for this purpose.
2. Description of the Related Art
There is described in Swedish Patent Specification No. 9800473-2 a method of cleansing waste gases, e.g., fumes or exhaust gases, from a gas burner or some other combustion source, such as an oil-fired burner or an internal combustion engine. The invention also relates to a gas burner in which said method is exploited.
It is common practice to heat industrial furnaces with the aid of gas burners. Natural gas is a typical fuel in this respect, although other gases may be used, for instance propane, butane, and LP-gas.
An effective gas burner is, for instance, one where the burner head is placed at one end of an inner tube which is surrounded by a protective tube that has a closed bottom. The fumes from the burner chamber pass inside the inner tube, down to the bottom of the outer tube, and there turn and flow up between the outer and the inner tubes in the opposite direction, and then into an exhaust channel that leads to the surroundings. The protective tube emits heat to a furnace space, this heat being 30% convection heat and 70% radiation heat.
A similar type of burner also includes an inner tube which is surrounded by an outer protective tube, but where the bottom of the protective tube is not closed. The protective tube extends in an arched shape, for instance a U-shape, where the free end of the protective tube is connected to an exhaust channel. The inner tube carrying the burner head is straight and is thus located within the straight part of the protective tube. Such gas burners emit high concentrations of hydrocarbons (HC) and nitrogen compounds (NOx).
It is desirable to maintain the outer tube at a temperature of up to 1150-1200°C., so as to enhance the power concentration of the burner. This can be achieved by constructing the outer tube from a high temperature material, such as from silicon carbide (SiC) or APM. APM is an acronym for Advanced Powder Metallurgy, this material comprising about 73% Fe, 22% Cr and 5% Al and which is extruded to a tubular shape.
However, the NOx concentration in the waste gases increases significantly at such high temperatures.
One problem occurring at high temperatures is that the catalyst is subjected to high thermal stresses, and hence a conventional catalyst that includes ceramic monoliths can be destroyed unless cooled.
The invention defined in the patent specification identified above affords a solution to these problems and results in much lower concentrations of CO and NOx, as well as HC, than those achieved with conventional catalytic waste gas purification processes.
The patent specification identified above relates to a method of cleansing from waste gases their nitrogen oxide (NOx), hydrocarbon (HC), and carbon monoxide (CO) contents, for instance burner fumes and internal combustion engine exhaust gases. The gases are caused to pass through a catalyst for catalytic purification of said gases. According to that patent specification, the lambda value is caused to lie beneath a value λ=1. The waste gas, or exhaust gas, is conducted through a first catalyst and thereafter through a second catalyst, wherein the CO-concentration of the gas is caused to be sufficiently high in the first catalyst to reduce the NOx, concentration to N2 to an extent such as to bring the NOx concentration down to a predetermined value, and wherein a sufficiency of oxygen is introduced between the first and the second catalyst such as to oxidize both CO and HC to CO2 and H2O to an extent such as to bring the CO-concentration down to a predetermined value.
One significant problem with practicing the invention according to said patent is that the lambda value cannot be measured directly, but must be measured indirectly.
This problem is solved by the present invention, which enables active regulation to be effected during operation of the system.